Use backtracking when solving dependency constraints.

utils/tests/pub/test_pub.dart

Index: utils/tests/pub/test_pub.dart
diff --git a/utils/tests/pub/test_pub.dart b/utils/tests/pub/test_pub.dart
index 156f61691350979adf19161cbf0f9c6ed9096b97..cce6d89ccf316f73519581b369996f3150073b15 100644
--- a/utils/tests/pub/test_pub.dart
+++ b/utils/tests/pub/test_pub.dart
@@ -22,6 +22,7 @@ import 'package:pathos/path.dart' as path;
 import 'package:scheduled_test/scheduled_process.dart';
 import 'package:scheduled_test/scheduled_server.dart';
 import 'package:scheduled_test/scheduled_test.dart';
+import 'package:unittest/unittest.dart' as unittest;
 import 'package:yaml/yaml.dart';
 
 import '../../pub/entrypoint.dart';
@@ -46,7 +47,7 @@ import 'descriptor.dart' as d;
 initConfig() {
   // If we aren't running on the bots, use the human-friendly config.
   if (new Options().arguments.contains('--human')) {
-    configure(new CommandLineConfiguration());
+    unittest.unittestConfiguration = new CommandLineConfiguration();
   }
 }